The present invention relates to a fuel injection system for air-compressing internal combustion engines, having an injection pump and an injection valve connected to one another by an injection line. The system is further comprised of a metering piston unit arranged before the injection valve and a time-delay member which is connected in parallel to the metering piston unit. The time-delay member is provided with a check valve that opens in the direction of the injection valve. The volume of a cylinder chamber of the metering piston unit corresponds to a pre-injection quantity of fuel.
According to U.S. Pat. No. 4,711,209 an injection system is known with which a division of the injection process into a pre-injection and a main injection is possible. For this purpose a metering piston unit is provided within the injection line, and connected in parallel thereto a time-delay unit is arranged. When a pressure wave generated by the injection pump is advancing in the injection line, it first reaches the metering piston unit and, against the force of a return spring, moves a metering piston, thereby supplying the injection valve with a fuel quantity that is delimited by the stroke of the metering piston and corresponds to the desired pre-injection. During the pre-injection step a check valve within the time-delay member is closed. Another portion of the pressure wave branches off before the metering piston unit via a time-delay member and reaches with a time delay, due to the difference in travel distance, the check valve and opens it against the force of a spring. Now the fuel from the time-delay line which corresponds to the main injection quantity may be injected via the injection valve into the combustion chamber. The time difference between the pre-injection and the main injection may be varied by the difference delay line. The main disadvantage of such an injection system is that, directly after the occurrence of the pressure wave impulse which initiates the main injection, the velocity with which the injection quantity increases during partial load and at low revolutions of the combustion engine is very low.
It is therefore an object of the present invention, for certain operational conditions of the combustion engine such as partial load and/or low revolutions of the engine, to substantially increase the velocity with which the injection quantity increases.